The energy crisis has generated considerable interest in the area of latent catalysts which may be activated to cure a polymer composition without a considerable expenditure of heat. Coating applications have received a major portion of this interest.
One of several techniques which have been proposed in this area involves dispersing an encapsulated catalyst throughout the uncured polymer coating composition and then rupturing the catalyst as the coating composition is sprayed onto the substrate. Typically, the shear forces exerted on the coating composition as it passes through a spraying nozzle are sufficient to rupture the microcapsule and release the catalyst. Other applications which may employ an encapsulated component are pressure sensitive inks and adhesives, flame retardant additives, two-stage resins and the like.
The use of encapsulated catalysts, cocatalysts and the like has not been totally successful in coating applications because of the difficulty of economically producing a suitably leakproof microcapsule. This is an absolute requirement in producing a storage stable composition when a highly active catalyst is encapsulated and dispersed in a polymerizable composition. Typical interfacial polymerization or coacervation methods of encapsulating catalysts in single-walled microcapsules simply have not proved to be effective methods of producing storage stable compositions which are exposed to solvents.
In response to this problem, efforts have been directed at developing dual-walled microcapsules which would meet the substantially leakproof criteria. Typically, leaks occur through pinholes, and if two walls are used the pinholes in each wall must be aligned to cause a leak. To date, such efforts have employed a combination of the interfacial polymerization technique to form the inner wall and a coacervation technique to subsequently form the outer wall. The major drawback of these particular techniques is obvious -- two distinct processes are required to form a dual-walled microcapsule and each process contributes to the total cost.